In motor drivers such as spindle motors of HDD devices, fans and pumps, small and highly-efficient permanent magnet motors (three-phase synchronous motors) are widely used.
However, in order to drive a permanent magnet motor, position information of a rotor of the motor is necessary, and therefore, a position sensor therefor has been needed. In recent years, the sensorless control, in which the number of rotations and torque of a permanent magnet motor are controlled without using the position sensor, has been widely spread.
By virtue of the practical realization of the sensorless control, the cost of the position sensor (for example, the costs of the sensor itself and wiring thereof) can be cut and the size of the device can be reduced. Moreover, since the sensor is not needed, there is an advantage that it can be used under a bad environment.
Currently, in the sensorless control of the permanent magnet motor, for example, a method in which an induced voltage (speed-induced voltage) generated by rotation of the rotor of the permanent magnet motor is directly detected and used as the position information of the rotor, thereby driving the permanent magnet motor, a position estimating technique in which the rotor position is estimated and calculated from a mathematical expression model of an intended motor and others are employed.
These sensorless controls also have a big problem. It is a position detection method during low-speed drive. Most of the sensorless controls which are currently in practical use are based on the induced voltage generated by the permanent magnet motor. Therefore, when the motor is stopped or in a low-speed range in which the induced voltage is low, the sensitivity is lowered, and the position information may be buried in noise. Various measures have been proposed against this problem.
The invention described in Japanese Patent Application Laid-Open Publication No. 7-245981 (Patent Document 1) is a method in which high-frequency power is distributed to a permanent magnet motor and the position of a rotor is detected from the current generated at that point. The rotor of the permanent magnet motor needs saliency, and the position detection is enabled by the influence of current harmonic waves caused by the salient-pole structure.
In the invention described in Japanese Patent Application Laid-Open Publication No. 2001-275387 (Patent Document 2), two phases of a three-phase stator coil are sequentially selected to apply a pulse-like voltage thereto, the pulse voltage induces the voltage of the phase to which the power is not distributed (in this case, transformer induced voltage), and the induced voltage is detected to estimate the position of a rotor from the voltage pattern thereof. This is for the reason that, since the saturation state of a magnetic circuit is varied by the position of the rotor, the induced voltage corresponding to the position is generated in the phase to which power is not distributed. Therefore, In the invention described in Patent Document 2, the position information can be acquired even in a completely stopped state.
In the invention described in Japanese Patent Application Laid-Open Publication No. 2003-189674 (Patent Document 3), the method described in Patent Document 2 is partially employed in the start-up (acceleration), and the acceleration is carried out reliably while checking the position of the rotor.
In the invention described in Japanese Patent Application Laid-Open Publication No. 2000-232797 (Patent Document 4), a “neutral-point potential” which is the potential of a connecting point of a three-phase stator coil is detected so as to obtain the position information. Although it takes the labor for leading of the neutral point of the stator coil, the position information can be obtained even when the three phases are in the power-distributed state at the same time. Therefore, a permanent magnet motor can be ideally driven by a sine-wave current.